Separate from the Gardaí, a Protection Officers Corps was set up to provide bodyguards for prominent treatyites during the Civil War. There was also a more secretive body called the Citizens Defence Force, responsible directly to Kevin O’Higgins and funded through the offices of Arthur Cox. In charge of it was a Captain Henry Harrison, a Englishman who had been a meddler in Irish politics for many years. The exact function of this force remains unclear but some believe responsible for the unsolved disappearance of a number of prominent Republicans at that time. The remains of one of them, Noel Lemass, was discovered by accident years afterwards; secretly buried in the Dublin Mountains.

Oriel House was taken over and became a much-feared interrogation centre. The Oriel House team, about 80-strong, was made up entirely of ‘ex-British officers’ and was accused of using brutal interrogation techniques and of the assassination of republican suspects and prisoners. A study of the period concluded, 'Oriel House succeeded in its task of suppressing small scale republican activities in the Dublin area, not by the sophistication and efficiency its intelligence work... but by the more direct method of striking terror into its opponents.[6] They were disbanded after the Civil War: supposedly because of squeamishness on the part of Kevin O’Higgins; but a core was retained as G-division of the Dublin Metropolitan Police: the G-men, headed by ex-RIC Inspector David Neligan.

In 1926, after the DMP had been amalgamated with the Gardaí, this unit was renamed the Special Branch in imitation of English nomenclature.

Gardaí use five levels to grade the international terror threat level to the Republic of Ireland; low, moderate, substantial, severe and critical. The gradings are based on a series of factors, including information supplied by international authorities. The current level is considered to be moderate, meaning an attack is "possible, but not likely".[11][12]

The SDU uses anti-terrorism legislation (chiefly the Offences against the State Acts) to conduct operations, and uses the non-jury Special Criminal Court to prosecute terrorism offenders. The SDU is a highly secretive organisation that does its utmost to stay out of the media spotlight, and when officers from the unit appear in court they routinely give evidence anonymously from behind a screen to protect their identity. All officers within the unit carry firearms, and train more often than regular police detectives. All officers are armed with concealed semi-automatic pistols, and if the situation requires, they have access to shotguns, submachine guns, assault rifles and sniper rifles. The SDU has a large pool of unmarked and armoured cars, and has access to helicopters if needed.

The budget for the SDU is classified, although it is drawn from the overall Garda budget (€1.34 billion in 2014). The only published figures are those for the budget to pay confidential informants, via the Covert Human Intelligence Sources (CHIS) system. The fund used to pay informers is known as the "Secret Service" budget, and it is shared between the Crime & Security Branch and Directorate of Military Intelligence. In 2014, it was €1 million.[14] Although the service's jurisdiction is the Republic of Ireland, officers from the unit have been active in Northern Ireland, a region which is officially part of the United Kingdom.[15]

In 2013, the Republic of Ireland had the fourth highest number of arrests for terrorism-related offences in the entire European Union, with 41 individuals arrested, behind only France (225), Spain (90) and the United Kingdom (77), according to Europol.[16] The average prison sentence for a person convicted under terrorism legislation in the Republic of Ireland was 12 years in 2013, the third highest in the EU, behind Greece (27 years) and Spain (14 years).[17]

The Garda Emergency Response Unit (ERU) is the national armed tactical intervention unit of the Gardaí, and a former subsection of the Special Detective Unit.[18] The ERU's manpower is approximately 100 full-time officers, who are trained and equipped for a range of situations which other Garda units may be unable to deal with. The SDU and ERU carry out joint operations.[19]

The Garda Counter-Terrorism International (CTI) unit was established in 2014 as a dedicated section of the SDU to identify and monitor Islamic terrorism threats and share intelligence with international security and law enforcement agencies on this issue. The Garda CTI has up to 50 full-time members who operate over 5 shifts. This section has its own surveillance teams and informants, and reports directly to the Garda Commissioner.[20] The Counter-Terrorism Domestic (CTD) section deals with homegrown terrorism.[1]

The Garda Hostage Negotiation Section (HNS), officially formed in 2007, is a subdivision of the Garda SDU. It is headed by an experienced Detective Superintendent, who cannot be identified for operational reasons. The senior HNS officer along with the Garda National Negotiator Co-Ordinator (NNC) are involved in the selection, training and coordination of the more than 70 specialist Garda negotiators stationed in all divisions in the country. Officers under the command of this section are not necessarily Special Branch detectives. Negotiators are educated at the Garda Síochána College, Metropolitan Police Service, Federal Bureau of Investigation (FBI) and the Central Mental Hospital (HSE National Forensic Mental Health Service).[21]

The Garda Witness Protection Unit is a small, dedicated and highly secretive subsection of the SDU, which operates the state's Witness Security Programme, at the direction of the Attorney General of Ireland. The unit (established in 1997) is headed by a Detective Inspector, and further reinforcements can be drawn from the Special Branch when there is a need. The Garda ERU provide tactical assistance to the programme. The budget for the unit was €1.198 million in 2014. Witnesses in the programme are given a new identity, address and armed police protection either in Ireland or abroad. There has never been a reported breach of security in which a protectee was harmed.[22]

An MI5 unit based at the regional headquarters (Palace Barracks) in Holywood, County Down, in Northern Ireland works with both the PSNI and Gardaí to track the activities of domestic and foreign terrorist threats, and in 2008 this led to the discovery of a suspected al-Qaeda bomb making factory in County Kerry, where three people were arrested, some of whom had previously been under surveillance by the Special Detective Unit's Middle Eastern Bureau[23] and National Surveillance Unit.[24]

SDU officers have conducted operations alongside Irish Military Intelligence, the United States' Central Intelligence Agency (CIA) and FBI in monitoring, arresting and prosecuting suspected al-Qaeda extremists using Ireland as a launchpad for terror attacks on Europe and the US.[25] The agency was involved in foiling several radical Islamic militants in Cork suspected of plotting to assassinate Swedish cartoonist Lars Vilks in 2009, in response to an al-Qaeda in Iraq bounty after the Muhammad drawings controversy, working with British and American intelligence agencies.[26][27]

The Special Detective Unit was the principal security agency involved in protecting Britain's Queen Elizabeth II on her historic visit to the Republic of Ireland in the summer of 2011, facing a significant threat from dissident republican paramilitary and terror organisations ("IRA"), and they scuppered a number of attempted bombings and assassination attempts to ensure a peaceful visit.[29][30] The SDU also worked alongside the United States Secret Service (USSS) and Diplomatic Security Service (DSS) in a heightened security state during the visit of US President Barack Obama to Ireland in 2011, fearing retaliation from al-Qaeda terrorists in response to the death of Osama bin Laden a number of weeks earlier. It was Obama's first trip overseas following the killing of bin Laden by US Navy SEALs in Pakistan, and his visit was deemed a major success.[31]

Following the outbreak of the Syrian Civil War (2011–present) and the campaign of violence perpetrated by the so-called "Islamic State" (2014) terrorist organisation in Iraq, the Middle Eastern Bureau of the Special Detective Unit and the Directorate of Military Intelligence increased their monitoring of Irish residents and citizens who travelled to conflict regions in the Middle East to partake in fighting, with the fear they may become radicalised and carry out Islamic jihadist terrorist attacks in Ireland, Europe and North America upon their return.[32] The Middle Eastern Bureau and Military Intelligence is tasked with monitoring between 30 and 60 people living in Ireland who are under suspicion due to their links with Muslim extremist organisations.[33][34] The Irish authorities have been working with the British security services, MI5 and the Secret Intelligence Service (SIS/MI6), to identify possible returning terrorists by sharing data, information and intelligence, operating a joint database and watchlist, and cross-checking airline passenger manifests to track Irish and UK residents and citizens who are travelling to and from conflict zones in the Middle East.[35] The US National Security Agency (NSA) also shares intelligence with the Gardaí and Defence Forces security services with regards to the activities of fighters in Syria and Iraq who have originated from the Republic of Ireland.[36]

It has been alleged that the Special Detective Unit has been involved in the telephone tapping and email hacking of crime journalists in Ireland, in attempts to reveal the identity of sources that appear in newspaper stories on crime and terrorism. A number of journalists have accused the unit of monitoring their electronic communications, as well as carrying out covert surveillance on them when they are meeting sources in person. Some journalists allege they have been questioned (and threatened with arrest) about sources used in crime and terrorism stories, including about Garda officers speaking to the media when not sanctioned to do so by senior management. Warrantless wiretapping by the authorities in the Republic of Ireland is not illegal, but in the absence of a warrant resulting evidence cannot be used to bring prosecutions in court. The press say they have been asked not to publish certain stories which may jeopardise sensitive Garda operations, but that the force has not attempted to influence reporting.[37]

In the month of January 2012 alone, Gardaí applied for 1,829 "requests for disclosure" from telecommunications and internet companies based in Ireland. There were 1,296 subscriber requests, 494 call trace requests and 39 IP (computer address) requests. Taken as an average monthly total, theoretically Gardaí would be seeking details of over 20,000 phone and broadband subscribers per annum. These requests have been the subject of an investigation by the Office of the Data Protection Commissioner.[38]

A number of officers in the Garda Special Detective Unit have lost their lives in the course of serving the state since the early 1920s.
Seven Special Branch detectives are known to have been unlawfully killed in the line of duty since 1940.[39]